We present the results of our systematic investigation of the RE dependency of superconductivity in the parent compounds T’-RE2CuO4 (RE = Pr, Nd, Sm, Eu, Gd, and Tb). Superconducting samples were prepared by metal organic decomposition (MOD). A stringent control of synthesis- and post-annealing-conditions is required to obtain superconducting samples. Superconductivity with a transition temperature (Tconset) ≥ 30 K is achieved for RE = Pr and Nd. By contrast, Tconset is at highest 20 K for RE = Gd. Our results indicate that the induction of superconductivity into T’-RE2CuO4 cuprates strongly depends on the RE3+ ionic size. This trend is discussed from the viewpoint of RE-dependent thermodynamic stability of T’-RE2CuO4. For smaller RE3+ ions, the thermodynamic boundary conditions become tighter.

We suggest a model explaining nonlinear dependences of critical current Jc in YBCO epitaxial films. Two features of YBCO are taken into account: twin domain structure in orthorhombic phase and the anisotropy of uniaxial strain dependence of Tc. Applied strain changes elementary pinning force of the defects located at low-angle dislocation boundaries between differently oriented twin domains. Account of Tc dependence on strain this leads to approximately parabolic strain behavior of Jc. We have obtained analytical expressions for the “initial strain”, which actually describes a natural misbalance between numbers of grain boundaries separating a and b oriented domains, as well as for “strain sensitivity”, which is determined by Tc dependence on uniaxial a and b strains and by the effective redistribution of vortices. Other experimentally observed effects, such as temperature, magnetic field and two-peak strain dependences of Jc, are shown to be described in the framework of suggested model.

We report on our growth of superconducting SmFeAs(O,F) films by F diffusion. In our process, F-free SmFeAsO films were grown by molecular beam epitaxy (MBE) first, and subsequently F was introduced into the films via F diffusion from an overlayer of SmF3. We performed a detailed comparison of the growth conditions and also the properties of resultant films for fluoride and oxide substrates. The best films on CaF2 exhibited a high transition temperature, Tcon (Tcend) = 57.8 K (56.4 K) at highest, which may exceed the highest Tc ever reported for bulk samples. Furthermore the films on CaF2 also showed high critical current density over 1 MA/cm2 in self-field at 5 K.

We report the microstructural features of GdBa2Cu3O7-δ (GdBCO) coated conductors (CCs) on LaMnO3 (LMO)-buffered IBAD MgO template, produced by the Reactive Co-Evaporation Deposition & Reaction (RCE-DR) process. Analysis results by X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that a lot of elongated round second phase particles of 70-150nm size within the GdBCO matrix were the Gd2O3 phase, a small amount of Cu-O phase were also trapped in the GdBCO matrix, and a thick layer of Cu-excessive Ba-Cu-O phase was found on the top surface of the GdBCO film, suggesting that the GdBCO film might be grown from Gd2O3 and liquid phase by a peritectic recombination. While both the GdBCO film and some Gd2O3 particles grown on the LMO-buffer layer were biaxially textured, the Gd2O3 particles fully trapped in the GdBCO matrix were randomly oriented. The Gd2O3 particles located at the interface between the GdBCO and LMO buffer layer exhibited the following crystallographic orientation relationship: LMO [010] // GdBCO [010] // Gd2O3 [110]; LMO [001] // GdBCO [001] // Gd2O3 [001].

We have modified the model of rotational relaxation of stresses at mismatched interface by taking into account elastic strains of the growing film. This extended the model validity range to a wider class of compounds including pnictides. The model describes formation of low angle boundaries consisting of threading edge dislocations. Calculated interface energy shows that rotational relaxation occurs due to finite size of clusters and to non-equilibrium effect of the film growth. Subgrain size and expected angle of domain rotation depending on the lattice mismatch have been estimated. Unusual effect of increasing angle between the film subgrains at reduction of the deposition rate is predicted. The computed parameters of subgrains are consistent with the observed film nanostructure.

We report the dependences of the degrees of tri- or bi-axial orientation on strength of applied magnetic fields of modulated rotating field (MRF) for twinned ErBa2Cu3Oy (Er123) powder samples oriented in epoxy resin under various MRF conditions. Introduction of a pulverization process in the Er123 powders improved the degrees of inplane orientations, and is effective for enhancing the inplane magnetic anisotropy of Er123 grains with twin microstructure. Formation of inhomogeneous domain structure is a dominant factor of the enhancement, and the present study indicates possibility of tri- or bi-axial orientation of the twinned Er123 grains even under relatively low MRF conditions around 1 T.

Epitaxial Fe-Te-Se thin films were deposited by pulsed laser deposition at 250 ~ 600 °C on SrTiO3 (100, STO), MgO (100), LaAlO3 (100, LAO) and CaF2 (100) single crystal substrates. Best superconducting film was grown on CaF2: Tconset = 20.0 K and Tc0 = 16.18 K with Tdep = 300 °C, 45000 pulses, 3 Hz. The critical current density Jc at 4.2 K was 0.41×106A/cm2 at 0 T and 0.23×106 A/cm2 at 9 T. Angular dependence of Jc showed broad c-axis correlated peak when B ≥ 3 T.

We present a first principles study of the electronic and magnetic properties of Fe-based pnicitdes superconductors as a function of pressure and doping. We show that the magnetic phase and a local magnetic moment persists at doping level quite larger than what found in experiments and the pressure phase diagram consists of a paramagnetic, antiferromagnetic and non-magnetic phases.

Although this result calls for the inclusion of long-wavelength or local fluctuations of iron magnetic moment and non-hydrostatic pressure effects, in order to improve the theoretical description of real experimental conditions, recent photoemission experiments[1] reconcile these DFT results, showing a local magnetic moment on Fe site different from zero in the paramagnetic, antiferromagnetic and the superconducting phase.

Changes in critical current properties depending on growth temperature (Ts) were clarified for Ba-Nb-O-doped YBa2Cu3Oy (Y123) films deposited by YAG- and excimer-PLD. Due to the introduction of Ba-Nb-O-nanorods, a vortex-Bose-glass-like behavior emerged as irreversibility lines and in-field critical current densities (Jcs) were improved. Crossover magnetic fields (Bcr) and in-field Jcs increased with the increase in Ts for the Y123 films with nanorods. These Ts-dependent critical current properties were attributable to the changes in morphology of the nanorods with Ts and were independent of laser source in PLD apparatuses. For the fabrication of RE123 coated conductors containing nanorods, optimization of Ts with taking both materials of RE123 matrix and nanorod into account is necessary to achieve higher in-field Jc.

Single-crystalline films of superconducting Sr1-xKxFe2As and Ba1-xKxFe2As2 were grown by molecular beam epitaxy (MBE). The most crucial problem in MBE growth of these compounds is the high volatility of elemental K. The key to incorporating K into films is low-temperature growth (≤ 350 ºC) in reduced As flux. We performed a systematic study of the doping dependence of Tc in Ba1-xKxFe2As2 for x = 0.0 to 1.0. The highest Tcon (Tcend) so far attained for Ba1-xKxFe2As2 is 38.3 K (35.5 K) at x ~ 0.3.

To enhance a global critical current in a superconductor, it is indispensable to understand current limiting factors and their influence on such a critical current. From this point of view, we have investigated in-plane distribution of local critical current density and its electric field criterion in a thin-film superconductor by using scanning-Hall probe microscopy. In a remanent state, after the application of sufficiently high magnetic field to a sample, current flows at critical current density according to the critical state model. Such distribution of current density was estimated from that of measured magnetic field using the Biot-Savart law. Furthermore, the corresponding electric field criterion was evaluated from the relaxation of such remanent magnetic field by considering Faraday’s law. This means that we could estimate in-plane distribution of local critical current density as a function of electric field criterion in a nondestructive manner. This characterization method would be very helpful for finding current limiting factors in a thin-film superconductor and their influence on its global current density versus electric field properties which would usually be obtained by four-probe method.